1. Field of the Invention
The present invention relates to a pulse width modulation controller and, more particularly, to a pulse width modulation controller for controlling a power switching means.
2. Description of the Related Art
Since the early development of the field effect transistor (FET), the FET has been typically used in low power applications where each transistor conducted relatively low levels of current, such as in Integrated Circuits (IC). The FET is now also used in high power applications, i.e. 100/200 v commercial voltage levels, where a FET switch is used to conduct high current levels. One of the circuits commonly used for controlling the on/off operation of a FET is a pulse width modulation controller.
FIG. 1 is a block diagram of a conventional pulse width modulation controller operating in a current mode. Referring to FIG. 1, there is an N-channel FET 13 operating as a switching means, two capacitors Cc and Ct, and resistors Rs and Rt. The conventional pulse width modulation controller 11 is an IC having external connections to the N-channel FET 13, the capacitors Cc and Ct and the resistors Rs and Rt. The pulse width modulation controller 11 for current mode includes an error amplifier 21, a comparator 23, an oscillator 25, an RS flip-flop 27, a NOR gate 29, and external contact pins 31-38.
When a feedback voltage Vfb is applied to the pulse width modulation controller 11, the error amplifier 21 compares the feedback voltage Vfb with a reference voltage Vref. The voltage output from the error amplifier 21 is divided by N using divider 41 and then applied to an inverting input terminal of the comparator 23. The comparator 23 then compares a voltage Vcs of the resistor Rs with the voltage output from the divider 41. Output Q of the RS flip-flop 27 is controlled by the voltage output from the comparator 23. The output of the NOR gate 29 is controlled by the output Q of the RS flip-flop and the output of the oscillator 25. The on/off operation of the N-channel FET transistor 13 is therefore controlled according to the voltage output from the NOR gate 29.
FIG. 2 is a block diagram of a conventional pulse width modulation controller 11 configured to operate in a voltage mode. In order to convert the conventional pulse width modulation controller from current mode operation to voltage mode operation, the pin 33 connected to the source of N-channel FET 13 in FIG. 1 must be connected instead to the capacitor Ct, as shown in FIG. 2. Thus, in order to be able to convert the conventional pulse width modulation controller 11 from current mode to voltage mode, the capacitor Ct must be external to the pulse width modulation controller circuit 11. If the resistor Rt and the capacitor Ct are integrated into the pulse width modulation controller circuit 11 in order to reduce the number of interface pins for the IC of the pulse width modulation controller 11, then the operational mode of the pulse width modulation controller 11 cannot be converted from current mode to voltage mode.
In addition, the pulse width modulation controller 11 operating in the current mode has a Zener diode 43 at an inverting input terminal of the comparator 23 which limits the current flowing through the N-channel FET 13. The Zener diode 43 maintains the voltage applied to the inverting input terminal of comparator 23 at a constant level. However, the voltage accumulated at capacitor Ct is much higher than the voltage limited by the Zener diode 43. Thus, the maximum duty cycle of the N-channel FET 13 is limited.
Conversely, in order to change the operation mode of the pulse width modulation controller 11 from voltage mode to current mode, the resistor Rs must be connected to the source of the N-channel FET 13. Also, the non-inverting input terminal of the comparator 23 must be disconnected from the capacitor Ct and connected with the source of the N-channel FET transistor 13. However, if the resistor Rs is integrated with the N-channel FET 13, then it is difficult to convert the operation mode of the pulse width modulation controller 11 from voltage mode to current mode.